1. Field of the Invention
The present invention relates to a laser welding method.
2. Description of Related Art
As one of welding methods for joining together a stack of a plurality of objects to be welded (e.g., metal sheets), a welding method employing laser welding is widely used for its advantages such as less processing distortion, high-speed welding capability, and fewer residual heat-affected zones. When performing laser welding, focusing a laser beam on a surface of a workpiece composed of a stack of a plurality of objects to be welded causes the diameter of the laser (laser diameter) on the surface (laser-radiated surface) of the workpiece to become smaller and the energy density of the laser to become higher. This may result in a defect, such as perforation, occurring in the workpiece.
In this regard, Japanese Patent Application Publication No. 61-238488 discloses a method in which spot welding is performed by radiating a laser beam to a stack of thin metal sheets. In the method according to JP 61-238488 A, the focal spot of the laser beam is set to be located on the near side at a distance, equivalent to the amount of defocusing, from a surface of the thin metal sheet. Thus, the depth of penetration can be set properly.
When laser welding is performed, the distance between a welding apparatus that radiates a laser beam and a workpiece (workpiece distance) can deviate (vary) from the intended distance (control-target value) due to various factors. Examples of the factors contributing to the deviation of the workpiece distance from the target value include inaccurate positioning of a welding robot, inaccurate robot teaching, displacement of the workpiece, and mechanical errors of the welding apparatus. If the workpiece distance thus varies, the laser diameter also varies. In other words, the laser diameter on the laser-radiated surface will not be stabilized.
In this case, if the laser diameter varies (deviates) in the direction of decreasing from the control-target laser diameter (the target value of the laser diameter), the energy density of the laser in the workpiece becomes too high. This may result in a defect, such as perforation, occurring in the workpiece. Conversely, if the laser diameter varies (deviates) in the direction of increasing from the target value, the energy density of the laser in the workpiece becomes too low. This may result in a lack of fusion into the object to be welded on the rear side (the opposite side from the laser-radiated surface) of the workpiece and, accordingly, in reduced weld strength. Thus, in the method of JP 61-238488 A, the energy density is not stabilized due to the unstable laser diameter, which may make it impossible to perform welding properly.